RESUMO
Recent amazing results (Nkolo et al., Org. Biomol. Chem., 2017, 6167) on the effect of solvents and polarity on the C-ON bond homolysis rate constants kd of alkoxyamine R1R2NOR3 led us to re-investigate the antagonistic effect of intramolecular hydrogen-bonding (IHB) on kd. Here, IHB is investigated both in the nitroxyl fragment R1R2NO and in the alkyl fragment R3, as well as between fragments, that is, the donating group on the alkyl fragment and the accepting group on the nitroxyl fragment, and conversely. It appears that IHB between fragments (inter IHB) strikingly decreases the homolysis rate constant kd, whereas IHB within the fragment (intra IHB) moderately increases kd. For one alkoxyamine, the simultaneous occurrence of IHB within the nitroxyl fragment and between fragments is reported. The protonation effect is weaker in the presence than in the absence of IHB. A moderate solvent effect is also observed.
RESUMO
Throughout the last decade, the effect of electron withdrawing groups (EWGs) has been known to play a role - minor or moderate depending on the nitroxyl fragment R1R2NO - in the change in the homolysis rate constant (kd) for C-ON bond homolysis in alkoxyamines (R1R2NOR). It has been shown that the effect of EWGs on kd is described by a linear relationship with the electrical Hammett constant σI. Since then, linear multi-parameter relationships f(σRS,ν,σI) have been developed to account for the effects involved in the changes in kd, which are the stabilization of the released radical (σRS) and the bulkiness (ν) and polarity (σI) of the alkyl fragment. Since a decade ago, new results have been published highlighting the limits of such correlations. In this article, previous multi-parameter linear relationships are amended using a parabolic model, i.e. (σI,nitroxide - σI,alkyl)2, to describe the effect of EWGs in the alkyl fragment on kd. In contrast to previous studies, these improved linear multi-parameter relationships f(σRS,ν,ΔσI2) are able to account for the presence of several EWGs on the alkyl fragment, R. An unexpectedly strong solvent effect - a ca. 1500-fold increase in kd - from tert-butylbenzene to the water/methanol mixture is also observed for 3-((2,2,6,6-tetramethylpiperidin-1-yl)oxyl)pentane-2,4-dione 1b in comparison to a ca. 5-fold increase in kd that is generally observed.
RESUMO
The design of new R1R2NOR3 alkoxyamines for various applications relies on the accurate prediction of two kinetic parameters, the C-ON bond homolysis rate constant (kd) and its re-formation rate constant (kc). Relationships to describe the steric and polar effects of the R1R2NO fragment ruling kd have been developed. For all cyclic nitroxyl fragments, the steric effect is described as the sum of the bulkiness of the R1 and R2 groups (i.e., normal steric effect), while for the noncyclic nitroxyl fragment (except for one case), a leveled steric effect is assumed. In this work, we show that the normal steric effect also applies to noncyclic nitroxyl fragments and that for one case an enhanced steric effect is also observed, i.e., experimental kd >5-fold larger than the predicted value.
RESUMO
In two recent articles (Org. Biomol. Chem., 2015 and 2016), we showed that changes in the phosphorus hyperfine coupling constant aP at position ß in ß-phosphorylated nitroxides can be dramatic. Such changes were applied to the titration of water in organic solvents and conversely of organic solvents in water. One of the molecules tested was a non-cyclic nitroxide meaning that a thorough investigation of the solvent effect on the EPR hyperfine coupling constant is timely due. In this article, we show that the aP of persistent non-cyclic ß-phosphorylated nitroxides decrease with the normalized polarity Reichardt's constant E(N)T. The Koppel-Palm and Kalmet-Abboud-Taft relationships were applied to gain deeper insight into the effects influencing aN and aP: polarity/polarizability, hydrogen bond donor properties, and the structuredness of the cybotactic region.
